1. Field of the Invention
The invention relates to a communication control apparatus that determines a base station to communicate with, and a communication apparatus and a communication system using the communication control apparatus.
2. Description of the Related Art
In recent years, cdma2000 1x - EV DO (hereafter referred to as “EV-DO”) has been developed as a next-generation high-speed radio communication system. The EV-DO is a system of specializing cdma2000 1x provided by expanding cdma2000 to the third-generation system for data communications and increasing the transmission rate. Here, “EV” means Evolution and “DO” means Data Only.
In the EV-DO, the configuration of a radio interface of an uplink from a radio communication terminal to a base station is almost similar to that in the cdma2000 1x. The radio interface configuration of a downlink from a base station to a radio communication terminal is the same as that in the cdma2000 1x in the band width stipulated to 1.23 MHz. On the other hand, it differs largely from that in the cdma2000 1x in the modulation system, multiplexing method, etc. The modulation system used in the cdma2000 1x is QPSK, HPSK; in the EV-DO, the modulation system is switched to QPSK, 8-PSK, or 16 QAM in response to the reception state of the downlink in a radio communication terminal. Consequently, if the reception state is good, a high transmission rate with low error tolerance is used and if the reception state is bad, a low transmission rate with high error tolerance is used.
TDMA (Time Division Multiple Access) rather than CDMA (Code Division Multiple Access) used in cdmaOne and cdma2000 1x is used as the multiplexing method for simultaneously conducting communications from one base station to a plurality of radio communication terminals. In the TDMA, the time is divided in 1/600 second units and communications are conducted only with one radio communication terminal in one time unit and further the radio communication terminal is switched every unit time for conducting communications with a plurality of radio communication terminals.
The radio communication terminal measures the Carrier to Interference power Ratio (CIR) of a pilot signal as the reception state of the downlink from the base station. The radio communication terminal also predicts the reception state at the next reception timing from the fluctuation in the CIR and sends the “receivable maximum transmission rate at a predetermined error rate or less” expected based on the prediction result to the base station as a Data Rate Control bit (DRC). Here, the predetermined error rate depends on the system design and usually is about 1%. The base station receives the DRC from a plurality of radio communication terminals and determines which radio communication terminal the base station is to communicate with in each time unit. To communicate with each radio communication terminal, basically the base station uses the high transmission rate as much as possible based on the DRC from the radio communication terminal.
In the EV-DO, the transmission rate of a maximum of 2.4 Mbps per sector is possible on the downlink. However, the transmission rate is the total of the data communication amounts with a plurality of radio communication terminals to which one base station is connected in one frequency band in one of sectors usually provided; if a plurality of frequency bands are used, the transmission rate is also increased.
JP-A-2002-300644 is known as a related art.
When the user conducts communications in the EV-DO in a building or a railroad vehicle, the strength of the radio signal received by the radio communication terminal may be insufficient or distortion of the radio signal may grow. In this case, generally the EV-DO transmission rate becomes low. Under such circumstances, an example of a method for improving the EV-DO transmission rate is to install a communication apparatus compatible with the EV-DO in a vehicle. The communication apparatus can communicate with an EV-DO base station installed outside the vehicle. Further, the communication apparatus includes a function as a base station in a wireless LAN system (hereafter referred to as “W-LAN”). The communication apparatus including the function can communicate with a W-LAN terminal used by the user in a vehicle. That is, the user can use a W-LAN terminal in a vehicle for connecting to the communication apparatus. The communication apparatus connected with the W-LAN terminal transmits and receives data between an EV-DO base station and the W-LAN terminal. If an antenna of such a communication apparatus is installed in a place of the vehicle where a good propagation environment is provided, the EV-DO transmission rate is improved.
To select a base station to communicate with, the communication apparatus generally measures the quality of each of radio transmission lines with a plurality of base stations (hereafter referred to as “transmission line quality”) and selects the base station with the best transmission line quality among them. If the base station selection method is applied to the case where the above-described communication apparatus is installed in each of vehicles, it is also possible that a plurality of communication apparatus may connect to a specific base station. In this case, the plurality of communication apparatus share the transmission band of the base station. Consequently, the transmission rate per communication apparatus is decreased.